Lignosulfonates are water-soluble materials. They are used in various applications and products, such as preparation of concrete admixtures, feed pellets, oil drilling muds, road stabilization, emulsion and dispersion stabilization, plant nutrition, leather tanning, dust collection, road de-icing and other applications. Lignosulfonates are metal or ammonium salts of lignosulfonic acids, and are either by-products of the sulfite pulping process, or products of sulfonation of other lignin derivatives. Lignin derivatives include, but are not limited to, kraft lignin, organosolv lignin, chemically modified lignin derivatives, and mixtures thereof.
Lignosulfonates can also be used to strengthen various substrates by treating them (e.g. coating, impregnating, etc.) with lignosulfonate solutions. Examples of these substrates are lignocellulosic-based substrates (such as paper, cardboard, and webs containing mixtures of lignocellulosic and polymer fibers). Note that for the purposes of this disclosure the terms "lignocellulosic-based substrates" and "substrates" will be used interchangeably. However, one major problem with using the lignosulfonates in the strengthening applications above is that they bleed off the substrates when contacted under humid conditions (e.g. touched with wet hands). This results in poor aesthetics, increased messiness, and poor strength retention. One method to reduce or eliminate this bleeding is to coat the treated substrates with wax or polymer films. However, this coating method is not an effective solution because the secondary coating materials are expensive to purchase, process and apply. Yet another method to reduce or eliminate the bleeding of the water-soluble lignosulfonates is to insolubilize them by crosslinking.
The crosslinking reactions for lignosulfonates that have been reported in the literature include the following: 1) condensation reaction with strong mineral acids at elevated temperatures (via the SO.sub.3.sup.2- units); 2) oxidative coupling reaction with hydrogen peroxide and catalysts (via the OH.sup.- groups); 3) reaction with bis-diazonium salts (via the .alpha.-position to the OH.sup.- groups); 4) reaction with bifunctional acid chlorides (via the OH.sup.- groups); 5) reaction with cyanuric chloride (via the OH.sup.- groups); 6) reaction with formaldehyde (via the CH.sub.2 groups); 7) reaction with furfural (via the .alpha.-position to the OH.sup.- groups); and 8) reaction with epichlorohydrin (via the OH.sup.- groups). However, the above reactions/processes include various processing problems, such as cost, low pH, long reaction times, harsh conditions (e.g. temperature), health hazards, etc.
What have been missing are simple and inexpensive methods to render lignosulfonates water insoluble, thus reducing or eliminating their bleeding from the lignosulfonate-treated substrates when contacted under humid conditions.